Method and apparatus for determining hearing characteristics



Jan. 15, 1963 J. A. VICTOREEN 3,073,900

METHOD AND APPARATUS F OR DETERMINING HEARING CHARACTERISTICS Filed May 16, 1958 n |o I I5 l I6 I l2 AMPLIFIER CLIPPER D GAIN l3 IS 25 7---l v I7 i; i I

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JOHN A.V|CTOREEN U- May/ ATTORNEY United States Patent? Gfiice 3,fi73,9ll Patented Jan. 15, 1963 3,073,900 METHOD AND APPARATUS FOR DETERMINHNG HEARING CHARACTERISTECS John A. Vietoreen, Colorado Springs, Colo. (1314 Druid Road, Maitland, Orlando, Fla.) Filed May 16, 1958, Ser. No. 735,897 7 Claims. (Cl. 179-1) This invention relates to a method and apparatus for limiting signals in an electrical circuit to a maximum amplitude without appreciably efiecting the amplification characteristics up to the limiting point and assuring that such limiting characteristics as exist are constant at all signal levels. The invention is particularly useful where a widely adjustable range of limiting is required and particularly to those cases where the range is greater than is possible to obtain satisfactorily by other methods.

Although the invention about to be described will be described in conjunction with its use in connection with electro-acoustic amplifiers and particularly hearing aids it will be apparent that it is also useful for other purposes such as electronic measurements and other places apparent to those versed in the art.

It is, as stated, particularly useful in connection with methods and apparatus for the fitting of hearing aids to individuals and more particularly for determining several of the important factors involved in fitting a hearing aid, determine the maximum tolerable peak pressure in the ear canal, and, the gain or amplification required, to raise useful sounds to the proper intelligible comfortable hearing level.

Heretofore, it has been well known, to those versed in the art, to provide hearing aids having different characteristics for different individuals. This was largely done by having on hand a supply of different types of aids and causing the individual to listen to different aids and to eventually select the one he liked best. This was a time-consuming process, highly unscientific and very often resulted in the individual selecting an aid which later proved to be unsatisfactory. Furthermore, it required the hearing aid dispenser or audiologist to have on hand a large and expensive stock of hearing aids, which only served to increase the confusion in the mind of the purchaser.

By the present invention it is possible to provide a simple method and apparatus for determining the more important parameters of the individuals hearing requirements and with this knowledge, a selection may be made from a minimum number of hearing aids to provide the individual with one having the proper characteristics to meet his requirements.

Briefly, the invention contemplates the provision of apparatus whereby sounds are transmitted to the ear of an individual through an earphone, an amplifier and a microphone or other calibrated sound source.

It should be pointed out particularly that a hearing aid is required to transmit sounds or signals at a level or with a gain sufficient to enable the user to obtain intelligibility without straining himself to obtain the intelligibilty. This normally means that a certain amount of minimum gain is necessary to transmit thenormal sounds desired to be heard. At the same time other sounds might be amplified, which were picked up at a higher level than the desired sounds and the reproduction of these sounds would be at such a high level that would be painful to the user of the aid, making it impossible for him to listen over any extended period of time.

Therefore, with the apparatus of the invention it is possible to select a hearing aid where the gain is such that intelligibility is realized and where the maximum amplitude of sound is cut oif at a certain level so that,

regardless of the sound picked up, the user will not be subjected to the painful explosions" due to high amplitude sounds being picked up and reproduced. Of greater importance is the fact that with the apparatus of this invention it is possible to allow the individual to listen through an apparatus which is adjustable, without interruption, to allow him to determine the characteristics of the hearing aid he will need, and then after this determination, because the device is calibrated, to select from a minimum number of hearing aids one which meets the requirements indicated on the testing means, much in the same manner as an oculist determines the type of lenses needed for sight correction.

In the drawings:

FIG. 1 is a schematic view of a system which may be used for carrying out the invention;

FIG. 2 is a more detailed view of a limiter or clipper stage; and

FIG. 3 is a view of a control panel for the instruments of FIG. 1.

As stated, in determining the correct hearing aid for a particular individual it is necessary to provide an amplifier which will afford sufficient gain to render the re ceived sounds or signal intelligible. Next, in order to prevent high level sounds from reaching the individuals car. it is necessary to provide a limiter which cuts off the high level sounds. Since the hearing of different individuals varies it is apparent that the gain required will vary and likewise the tolerance to loud sounds may vary.

Therefore, as shown in FIG. 1, I provide a diagnostic instrument which includes a microphone or a sound generator it} connected to an amplifier 11 having a gain control 12. The gain control obviously may be any desired suitable place in the amplifier. The signals having the gain controlled, are fed into a limiter or clipper 13 having an attenuator 13' in the input. In this circuit the maximum instantaneous amplitude of the signals are limited to a maximum predetermined value. It is apparent that the clipper could also amplify, depending upon the type. The output of the clipper is provided with an attenuator 14 having output terminals 15 connected to an earphone 16. The attenuators 13 and 14 are mechanically connected together, as indicated at 17, and act in the opposite directions to each other so that as attenuation in one is increased the attenuation in the other is decreased.

The value of the signal to the earphone is under control of a calibrated gain control which indicates by its setting the overall acoustic gain of the system. It will be apparent that the calibration of the gain control can be in decibels or some arbitrary unit which may be correlated with the instruments being dispensed. Likewise, the attenuators 13 and 14 may be calibrated in pressure units or in arbitrary figures correlated with the instruments being dispensed.

The limiter or clipper is a particularly critical part of the system. The clipper of the present invention should be one that provides a sharp clipping action and with a minimum of distortion up to and particularly near the clipping point. As previously stated, the clipping of the signal may be at different amplitudes depending upon the individual. In actual practice it has been found that this range may extend from to db and could be under 100 db or even less. The point of clipping should be accurately determined since the range between intelligibility determined by gain and tolerability determined by clipping in some individuals is very small and'may be only a few db apart. The clipper should be such that any loss or gain therein is known and is taken into consideration as a part of the loss or gain of the'system.

No particular problem is involved where a' clipper is operated at high levels, but when the clipper has to be variable and may be required to clip at a low level, at low level clipping the distortion at or near the clipping point is highly undesirable. This is particularly true when dealing with transistor circuits.

Normally clipping or limiting means is for limiting the maximum attainable output pressure at the ear drum. This is attained by saturation or overloading or by limiting actions of a diode limiter. In the saturation type devices the peak clipping is not attained clearly but as they reach saturation a non-linearity occurs near the peak. If the device is a high voltage device this undesirable effect is not particularly noticeable but it does become more noticeable as the voltage is reduced.

By the present invention clipping is effected at a sufficiently high operating level to overcome the distortion effects just prior to clipping normally present at lower level clipping. This is effected in this instrument by clipping at a constant level to maintain constant clipping characteristics. I11 effecting this the signal from the ampliher is fed into the attenuator 13' under control of the gain control 12. The signal is then limited by the clipper which may be a pair of diodes 20 biased by a voltage source 21 connected across the line to provide equal full wave clipping. The resistor 18 in series with the attenuator resistance 13' assures that the signal level into the clipper will always be large enough to provide a signal through the system. After the signal is clipped it passes into the load through the resistance of the attenuator 14 at the output of the clipper. It likewise has a resistance 19 in series with it, thus making sure that the signal cannot be reduced to zero. It should be pointed out that in actual practice and in order to make sure that the attenuators 13' and 14 each add or subtract resistance in equal amounts, that tapped resistances may be used.

Since the limiter portion of the circuit always limits at the same amplitude level of the signal, it will be appreciated that this level can ordinarily be so high that the slight distortion at the point of limiting is of no consequence. By providing the two attenuators in combination with the limiter, the apparent limiting in this system may be varied between widely different amplitudes, since the loss by the first attenuator is offset by the reduction in loss in the second attenuator and these two attenuators operating in unison and in opposite directions, they may be calibrated in terms of sound pressure.

The pair of attenuators always result in a loss in signal level. This may be compensated for in either the main amplifier 11 or preferably by providing clipper and amplifier combinations. The combination of the attenuators in conjunction with the limiter is particularly useful in that changes in the attenuator settings do not alter the overall acoustic amplification in the system although the apparent clipping amplitude of the signal may be altered and vice versa.

Since the frequency response characteristics of the input and output transducers of a system are not necessarily fiat in their frequency response it may be desirable to insert a compensating network 25 between the output of the attenuator 14 and the sound reproducing transducers. Where several different output transducers are to be used, a switch 26 may be provided permitting the frequency response to be altered for each transducer. In some cases it may be desired to have a frequency response that is not equal or fiat at all frequencies, in which case the compensating network may have any combination of frequency response that may be required or desired.

As an example, one manner in which the above described apparatus may be used for determining the proper type of hearing aid to furnish an individual is as follows:

First, a signal is amplified in the amplifier to a point where the system clips the signal, at this time the clipping or limiting control is placed at a position where clipping is at lowest signal level at which clipping is desired.

The gain of the system is then adjusted for maximum intelligibility on low level sound. The sound level to the microphone is then raised and the clipping adjustment is raised to increase the maximum output of the system until the point is reached where the output level of the increased sound is uncomfortable. The clipping level is then lowered to determine the pressure to which the final aid shall be used.

All of the foregoing is done at one setting of the compensating network, which incidentally may be set at a non-compensating position, and the above then repeated for other positions of the compensating network to determine the characteristic of the earphone desired or the most desirable frequency response.

It will thus be apparent that I have provided an instrument where an individuals hearing may be quickly and easily analyzed from the standpoint of his requirements in a hearing aid as to gain, maximum sound pressure and frequency response without the necessity of a long and laborious picking and chasing of the various components. Once the requirements are determined, a hearing aid meeting the requirements may be selected from a relatively small stock, the proper earphone selected, and the combination furnished to the purchaser.

It is especially important to note that a. versatile clipper is provided which does not effect the signal translation characteristic other than to determine the maximum output, which may be determined within wide limits, without effecting the usable signal.

It will appear, to those versed in the art, that by the addition of an output indictor on the output side of the gain control 12 the sound level of signals which are below the limiting value may be measured.

Having thus described the invention I am aware that numerous and extensive departures may be made therefrom by those versed in the art without departing from the spirit of the invention as defined in the appended claims.

I claim:

1. A device for limiting the maximum pressure transmitted by an electro-acoustic device consisting of an earphone, an amplifier to drive the earphone and limiter means consisting of a pair of attenuators coupled together with a limiting amplifier having a limited output between the attenuators so arranged that the maximum signal level through the limiting means remains constant when the combined attenuators are changed.

2. A device for limiting the maximum pressure transmitted by an electro-acoustic device consisting of an earphone, an amplifier to drive the earphone and limiter means consisting of a pair of attenuators coupled together operable in equal and opposite directions with a limiting amplifier having a limited output between the atttenuators so arranged that the maximum signal level through the limiting means remains constant when the combined attenuators are changed.

3. A device for limiting the maximum pressure transmitted by an electro-acoustic device consisting of an earphone, an amplifier to drive the earphone and limiter means consisting of a pair of attenuators coupled together operable in equal and opposite directions with a limiting amplifier therebetween having a limited output between them so arranged that the maximum signal level through the limiting means remains constant when the combined attenuators are changed and frequency compensating means provided between the limiter means and the earphone.

4. A device for limiting the maximum pressure transmitted by an electro-acoustic system consisting of an earphone, a limiting device to limit the electrical voltage to drive the earphone and limiter means consisting of a pair of attenuators coupled together with the limiting device between them and so arranged that the signal level through the limiter means remains constant when the combined attenuators are changed.

5. An apparatus for testing the bearing of an individual including a variable gain amplifier, means for limiting the signal amplitude from the amplifier and an earphone connected thereto, said limiter means including a clipper circuit for clipping a signal operable at a predetermined level, an attenuator at the input of the clipper circuit and a second attenuator at the ouput of said circuit, said attenuators being operable to change the signal at the input and output in opposite directions whereby the signal from the amplifier to the earphone is unchanged except for maximum amplitude.

6. An apparatus for testing the hearing of an individual including a variable gain amplifier, means for limiting the signal amplitude from the amplifier and an earphone connected thereto, said limiter means including a clipper circuit for clipping a signal operable at a predetermined level, an attenuator at the input of the clipper circuit and a second attenuator at the output of said circuit, said attenuators being mechanically coupled together and being operable to change the signal at the input and output in opposite directions whereby the signal from the amplifier to the earphone is unchanged except for maximum amplitude.

7. An apparatus as described in claim 6, wherein an adjustable frequency compensating network is included in the circuit between the earphone and the attenuator.

References Cited in the file of this patent UNITED STATES PATENTS 2,342,238 Barney Feb. 22, 1944 2,553,674 Purington May 22, 1951 2,605,355 Foster July 29, 1952 2,615,999 Culicetto Oct. 28, 1952 2,662,940 Barron Dec. 15, 1953 2,679,626 Miller May 25, 1954 2,849,546 Martin Aug. 26, 1958 

6. AN APPARATUS FOR TESTING THE HEARING OF AN INDIVIDUAL INCLUDING A VARIABLE GAIN AMPLIFIER, MEANS FOR LIMITING THE SIGNAL AMPLITUDE FROM THE AMPLIFIER AND AN EARPHONE CONNECTED THERETO, SAID LIMITER MEANS INCLUDING A CLIPPER CIRCUIT FOR CLIPPING A SIGNAL OPERABLE AT A PREDETERMINED LEVEL, AN ATTENUATOR AT THE INPUT OF THE CLIPPER CIRCUIT AND A SECOND ATTENUATOR AT THE OUTPUT OF SAID CIRCUIT, SAID ATTENUATORS BEING MECHANICALLY COUPLED TOGETHER AND BEING OPERABLE TO CHANGE THE SIGNAL AT THE INPUT AND OUTPUT IN OPPOSITE DIRECTIONS WHEREBY THE SIGNAL FROM THE AMPLIFIER TO THE EARPHONE IS UNCHANGED EXCEPT FOR MAXIMUM AMPLITUDE. 